With my M I used Image Capture to set the default app to launch for my SD cards. I had to do this only once per card and the system would remember not to launch Photos but Lightroom. With the SL this is no longer feasible since the camera's formatting algorithm seems to be more aggressive -- after formatting the card in the camera the card switches back to the default behavior. I have found this little terminal command to disable app launch on SD card insertion completely:
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool YES
While this doesn't allow me to pick a different app to launch I at least don't have to keep quitting Photos. This has worked for me on OS X 10.11.16. Use at your own risk!